1. Field of the Invention
The present invention relates to a flow meter for monitoring liquid flow at low or intermittent rates, particularly waste water gravity flow from a reverse osmosis system.
2. Description of the Prior Art
Prior art flow meters for measuring relatively low rates of liquid flow are typically vertically oriented, with the liquid whose flow rate is to be measured being admitted under pressure at the bottom of the meter. The meters usually include a moving part, such as an impeller, bobbin, ball or float, which is acted upon and raised by the upwardly flowing liquid. In one form of meter a float is movable vertically along a guide rod, the extent of vertical float travel being indicative of the liquid flow rate.
This type of prior art meter requires a significant amount of constant or continuous liquid flow, usually in the order of 10 or more ounces per minute, to move the float upwardly along the guide rod. The very low, sometimes discontinuous droplet flow of liquid through the usual reverse osmosis (RO) waste water exit conduit would be ineffective to raise the float. Attempted measurement of a typical RO droplet flow rate of 1/2 to 3 ounces per minute therefore would not be practical or particularly accurate. In addition, since RO droplet flow is normally a gravity induced flow into a vented chamber, the prior art float type meters dependent upon pressure would not be appropriate, and would be totally unsuited to measurement of RO waste water flow.
Other problems with the prior art float type meter are that they are typically expensive, and many do not provide linear scale readings.
There is a need in the prior art for a meter capable of continuously monitoring the rate of flow of RO waste water. Continuous monitoring is important for a user to determine if the RO system is operating properly, particularly whether maintenance or replacement of a component is needed. For example, a predetermined waste flow rate could be established which would indicate unacceptable clogging of the RO system membrane element that it should be cleaned or replaced.
For domestic or non-commercial RO waste water systems, the waste water flow is about 1 to 3 ounces or approximately 30 to 90 milliliters per minute. If a lower rate of flow is experienced it could indicate that there is a tear in the membrane or a failure of some other upstream system component that is allowing untreated water to bypass the waste water drainage line.
There are other reasons for the use of an accurate, low cost flow meter. Certain types of RO units are characterized by periodic high levels of waste water flow at predetermined time intervals to provide a "fast flush" mode. The flow may be increased to perhaps 50 ounces or about 1500 milliliters per minute. This produces a relatively turbulent flow of waste water across the RO membrane, which tends to dislodge or scrub away the typical membrane accumulation of contaminants that are relatively unaffected by the normal drip flow rate of RO waste water. It is important to be able to determine and monitor the establishment and duration of such increased flows to insure proper operation of the RO system in a fast flush mode.
As previously mentioned, most prior art flow rate meters require an upward flow of the liquid against a float or the like. The position of the float is indicated by markings or graduations provided on the meter at suitable vertical intervals. However, as noted earlier, such graduations are frequently not linear, that is, they are not uniformly spaced apart to show equal increments of flow rate increase. For example, a graduation located midway between a 4 ounce graduation and a 5 ounce graduation does not necessarily indicate a rate of flow of 4 and 1/2 ounces per minute.
This makes it difficult for a user to accurately determine the rate of flow for some float position located between two markings. The need to interpolate between such markings becomes more critical with a smaller domestic RO system, for example, in which the waste water output is between 1 and 2 ounces per minute versus a larger system output of perhaps 90 to 100 ounces per minute. Unless the meter is a more expensive custom designed version to render it linear, the user, maintenance or service technician must employ tedious and time consuming measurement alternatives such as the use of a measuring cup to collect liquid flow over a measured time period, from which the rate of flow can be calculated.
In addition to their inability to measure very low rates of flow, currently available flow meters are particularly unsatisfactory in determining the average rate of flow of an intermittent flow such as the droplet flow in domestic or household RO systems.
Such prior art flow meters also are generally available in one of several fixed scale ranges, such as 1 to 10 or 10 to 100 or 4 to 40 etc, forcing a user to select one best suited to the application. However, these would be inadequate to measure flow in much higher ranges, as in the RO systems having a fast flush feature.